1. Field of the Invention
The present invention relates generally to improvements in a radio pager and more specifically to a method and apparatus via which a message receive ratio can be improved by reducing intermodulation noise induced in a radio pager.
2. Description of the Related Art
In order to meet the ever increasing number of pager subscribers in a radio paring system, a large number of radio channels (viz., frequencies) have been put into practical service. Further, a recent trend of introducing free competition in the radio paging business has brought about the possibility of an accelerated increase in the number of paging channels taking place.
With such an increase in the number of frequencies to be used in the same service area, a pager is liable to fail to receive a message(s) directed thereto due to intermodulation noise resulted from two high level carriers with different frequencies.
Throughout the instant disclosure, a desired frequency implies the frequency used to transmit a message to a pager in question, while an undesired or interfering frequency indicates the frequency which may cause intermodulation noise with the desired frequency. It is to be noted that the undesired or interfering frequency is in fact a desired frequency to a group of other pagers.
Even if a pager is located in a place wherein a desired frequency wave has electric field strength sufficient for a message to be received, it is often the case that the pager fails to receive the message if an undesired frequency wave also has a very high electric field strength.
One known approach to solving the aforesaid problem, is to automatically lower a RF signal level in response to the field strength of an incoming RF signal. As is known in the art, an attenuator, it not energized, passes therethrough a signal without attenuation. In other words, the attenuator always requires electric power to perform its attenuation operation. If attenuation is not needed, power is cut-off to the attenuator. It follows that the above prior art has encountered the problem that a battery life is undesirably shortened. In more specific terms, the attenuator is energized irrespective of whether or not an incoming signal is a desired signal. This means that the attenuator operates even when a desired signal is transmitted.
Additionally, if the auto gain control loop is not precisely adjusted, a desired frequency signal with a normal level can be undesirably lowered by the attenuator. This may lead to lowering of message receive ratio.